Development of pressure or vibration sensing devices for area security is progressing at a rapid rate. Cruder forms of these intrusion sensors have demonstrated a certain degree of success. Usually the intrusion sensors assume the form of an elongate, hose-like structure having a number of vibration sensitive devices located along its length. A noteworthy example of a system of this type is disclosed in U.S. Pat. No. 3,806,908 issued to Lloyd R. Bound on Apr. 23, 1974. His system uses a number of piezoelectric transducers and wires contained in or connected to a buried cable. Another pair of interesting approaches is disclosed in the U.S. Pat. No. 3,832,704 issued to Vahram S. Kardashian on Aug. 27, 1974 and the U.S. Pat. No. 3,882,441 issued to James O. Holman et al on May 6, 1975. Both approaches rely on the interaction of magnetostrictive film plated wires to sense external disturbances and a location of a possible intruder. Still another intrusion detection system was designed by Robert F. Bell et al and patented on Sept. 3, 1974 in U.S. Pat. No. 3,833,897. A coaxial electrical cable converts mechanical vibrations to representive signals capable of being monitored at a remote station. The signals generated are distinctive sounds made by an intruder either climbing or cutting through a chain link fence. Provision is made for a loud speaker or an alarm generator to indicate where an intrusion has occurred.
All of the aforeidentified detection systems represent meritorious advances in the state-of-the-art; however, they all are bulky and tend to be expensive and preclude their use in a vast number of situations.
One seismic sensor that could lend itself to adaption as an intrusion sensor is disclosed in U.S. Pat. No. 3,798,474 issued to Jean Cassand et al and which is entitled Pressure Wave Piezoelectric Sensor of Continuous Structure. The pressure sensitive element is formed of a flexible strip of a piezoelectric plastic material for sensing seismic disturbances. The most common embodiment uses one or more of the pressure sensitive elements, each extending a considerable length to sense remote seismic disturbances. A modification reverses the polarities of the side-by-side longitudinally extending pressure sensitive elements to nullify the parasitic charges attributed to longitudinal flexure. Because of the length and electrical coupling of the pressure sensitive elements it is doubtful that it could differentiate between remote disturbances and those within a few feet of a buried or immersed intrusion device.
Thus, there is a continuing need in the state-of-the-art for an intrusion detector that is not unduly bulky, is not too sensitive to make it useless for close-in detection, is not too expensive and is relatively easily stored and deployed.